Process for the electrolytic copper plating of zinc diecasting having a reduced tendency to blister formation

ABSTRACT

In the electroplating of zinc diecastings with a copper layer, the electrolyte penetrates into the pores of the zinc diecasting. When the temperature is increased later, this leads to vaporization of the electrolyte liquid in the pores and to blistering or flaking of the copper layer. It is proposed that plating be carried out in two steps. In the first step, only a thin copper layer of less than 1 μm is applied and the plated parts are then treated at a temperature which leads to vaporization of the electrolyte liquid. The thin copper layer is still sufficiently porous for the vapour to be able to escape. Only the solid constituents of the electrolyte remain. The copper layer is then thickened to a final thickness of from about 20 to 30 μm. In this plating step, electrolyte liquid no longer penetrates into the pores of the zinc diecasting. The parts which are coated in this way display no blistering or flaking of the copper layer after storage at a temperature of 150° C.

The invention relates to a process for the electrolytic copper plating of zinc diecasting having a reduced tendency to blister formation.

The deposition of copper on zinc articles or zinc diecastings is adequately known to those skilled in the art (ref 4).

The first step in the copper plating of zinc diecastings is, according to the prior art (refs 1, 2, 3), the deposition of copper from an alkaline cyanide electrolyte. Subsequently, a bright copper layer from an acidic electrolyte or a nickel or bronze layer is usually deposited.

A particular difficulty in the plating of zinc diecastings is the structure of the base material formed on injection moulding. The casting is coarsely crystalline and permeated with voids in the interior. Only a thin outer layer is dense and pore-free. The outer layer is formed during injection moulding by rapid cooling of the melt on the walls of the casting mould. Only this outer casting skin can be electroplated according to the prior art. However, the casting skin is very sensitive and is sometimes chemically attacked and damaged during the pretreatment by degreasing and pickling, so that the pores of the base material are sometimes exposed. The plating baths themselves can also damage the casting skin.

A firmly adhering coating can no longer be applied to the damaged surface. In addition, blister formation frequently occurs as a result of the pretreatment baths or the electrolyte penetrating through the damaged casting skin into the pores of the base material. During later heat treatment, the liquid which has penetrated in vaporizes and pushes the applied coating outward to form blisters or raised regions. In unfavourable cases, the copper layer flakes off.

Further disadvantages of the processes described in the prior art lie in the use of highly toxic electrolytes. For occupational hygiene and environmental reasons, an alternative electrolyte composition therefore appears to be advisable.

It is an object of the present invention to provide a process for the electrolytic copper plating of zinc diecastings by means of which the above-described disadvantages of the prior art can be largely avoided.

This object is achieved by a process which comprises the following process steps:

-   -   a) deposition of a first copper layer having a thickness of less         than 1 μm from a pyrophosphate-containing copper electrolyte,     -   b) rinsing and drying of the parts at elevated temperature and     -   c) thickening of the first copper layer in a         pyrophosphate-containing copper electrolyte to a thickness in         the range from 10 to 20 μm.

According to the invention, a thin copper layer having a thickness of less than 1 μm is firstly deposited from a pyrophosphate-containing copper electrolyte on the casting skin of the zinc diecastings. In this first plating step or even during the pretreatment by degreasing and pickling, the casting skin is generally damaged. As a result, the electrolyte can penetrate into the now open porous microstructure of the zinc diecasting during the pretreatment or during plating. It is therefore of great importance to the process that the copper layer applied in the first plating step still has sufficient porosity for the carrier liquid of the electrolyte which has vaporized during the subsequent heat treatment to be able to escape. This layer should therefore be no thicker than 1 μm and preferably has a thickness in the range from 0.1 to 0.5 μm, in particular from 0.2 to 0.3 μm.

After the first plating step, the parts are rinsed and dried by storage at a temperature of from 100 to 180° C., preferably from 120° C. to 160° C. and particularly preferably about 140° C., for a sufficient time, e.g. from 10 to 60 minutes. In this heat treatment, the carrier liquid of the electrolyte which has penetrated into the porous zinc vaporizes. The copper layer present as a result of the first plating step is, because of its low thickness, still porous and not impermeable to the vapour being formed, so that the vapour formed on heating can escape. Only the solid constituents (salts) of the electrolyte remain in the pores and these do not interfere further. The presence of residual electrolyte salts can be confirmed, for example, by means of SEM and/or EDX studies.

Rinsing is preferably carried out using water and is adequately known to those skilled in the art.

After the plated and dried zinc diecastings have cooled, plating is continued, if appropriate in the same pyrophosphate-containing electrolyte, until from about 5 to 50 μm, preferably from 10 to 30 μm, particularly preferably from 10 to 20 μm, of copper have been deposited. The thin copper layer which is already present, at the beginning of the second plating step obviously prevents penetration of liquid electrolyte into the porous zinc base material. The parts which have been plated in this way withstand storage at 150° C. for about 30 minutes without blister formation or even flaking.

The copper layer in the first substep a) can be deposited by means of an electrochemical process. Electrolytic deposition (ref 4) is possible here. The second copper coating can be deposited reductively or preferably by means of an electrolytic process. Among electrolytic processes, essentially 3 different plating processes may be mentioned:

-   -   1. Drum plating for bulk material and mass-produced parts: in         this plating process, relatively low working current densities         are employed (order of magnitude: 0.05-0.5 A/dm²)     -   2. Rack plating for individual parts: in this plating process,         medium working current densities are employed (order of         magnitude: 0.2-5 A/dm²)     -   3. High-speed plating for tapes and wires in continuous plants:

in this plating sector, very high working current densities are employed (order of magnitude: 5-100 A/dm²)

The first two plating processes (drum and rack) tend to be of importance for plating with copper, with either drum plating (low current densities) or rack plating (medium current densities) being possible depending on different electrolyte types.

The application of the copper layer to the zinc diecasting in both process steps a) and c) is, as mentioned above, advantageously carried out by means of an electrolytic process.

Here, it is important that the metal to be deposited is constantly kept in solution during the process, regardless of whether electroplating is carried out in a continuous process or a discontinuous process. To ensure this, the electrolyte according to the invention contains pyrophosphate as complexing agent.

The amount of pyrophosphate ions present in the electrolyte can be set in a targeted manner by a person skilled in the art. It is limited by the fact that the concentration in the electrolyte should be above a minimum amount in order to be able to bring about the abovementioned effect to a sufficient extent. On the other hand, the amount of pyrophosphate to be used is guided by economic considerations. In this context, reference may be made to EP1146148 and the relevant information given therein. The amount of pyrophosphate to be used in the electrolyte is preferably 50-400 g/l. Particular preference is given to using an amount of 100-350 g/l of electrolyte, very particularly preferably about 200 g/l of electrolyte. The pyrophosphate can, if it is not introduced as salt constituent of the metals to be deposited, be used as alkali metal diphosphate or alkaline earth metal diphosphate or as H₂P₂O₇ in combination with an alkali metal or alkaline earth metal carbonate/hydrogencarbonate. Preference is given to using K₂P₂O₇ for this purpose.

In the electrolytes used, the copper to be deposited is present in solution in the form of its ions. They are preferably introduced in the form of water-soluble salts which are preferably selected from the group consisting of pyrophosphates, carbonates, hydroxycarbonates, hydrogencarbonates, sulphites, sulphates, phosphates, nitrites, nitrates, halides, hydroxides, oxide-hydroxides, oxides and combinations thereof. Very particular preference is given to the embodiment in which the copper is used in the form of the salts with ions selected from the group consisting of pyrophosphate, carbonate, hydroxycarbonate, oxide-hydroxide, hydroxide and hydrogencarbonate. What salts are introduced in what amount into the electrolyte can be decisive for the colour of the resulting layers and can be set according to customer requirements. The ion concentration of copper can be set in the range from 5 to 100 g/l of electrolyte, preferably from 10 to 50 g/l of electrolyte. The resulting ion concentration is particularly preferably in the range from 15 to 30 g/l of electrolyte. About 15-20 gram of copper per litre of electrolyte are very particularly preferably used, with the copper being introduced as pyrophosphate, carbonate or hydroxycarbonate salt into the electrolyte.

The pH of the electrolytes is in the range from 6 to 13 required for electroplating. Preference is given to a range of 6-12 and very particularly preferably 6-10. A pH of about 7.9-8.1 is most preferably employed.

Apart from the metals to be deposited and the pyrophosphates used as complexing agents, the electrolytes can contain further organic additives which perform functions as brighteners, wetting agents or stabilizers. The electrolyte according to the invention can also dispense with the use of cationic surfactants. The addition of further brighteners and wetting agents is preferred only when the appearance of the layers to be deposited has to meet specific requirements. The addition of one or compounds selected from the group consisting of monocarboxylic and dicarboxylic acids, alkanesulphonic acids, betaines and aromatic nitro compounds is preferred. These compounds act as electrolyte bath stabilizers. Particular preference is given to using carboxylic acids, alkanesulphonic acids, in particular methanesulphonic acid, or nitrobenzotriazoles or mixtures thereof. Suitable alkanesulphonic acids are mentioned in EP1001054. Possible carboxylic acids are, for example, citric acid, oxalic acid, gluconic acids, etc. (Jordan, Manfred, Die galvanische Abscheidung von Zinn und Zinnlegierungen, Saulgau 1993, page 156). Betaines to be used are preferably those from WO2004/005528 or from Jordan, Manfred (Die galvanische Abscheidung von Zinn und Zinnlegierungen, Saulgau 1993, page 156). Particular preference is given to those disclosed in EP636713. In this context, very particular preference is given to using 1-(3-sulphopropyl)pyridiniumbetaine or 1-(3-sulphopropyl)-2-vinylpyridiniumbetaine.

The electrolyte according to the invention is characterized in that it is free of hazardous substances classified as toxic (T) or very toxic (T⁺). It contains no cyanides, no thiourea derivatives and no thiol derivatives.

The deposition of the copper layers can be operated at a temperature chosen on the basis of the general knowledge of a person skilled in the art. Preference is given to a range from 20 to 60° C. within which the electrolytic bath is maintained during the electrolysis. More preference is given to a range of 30-50° C. The deposition is most preferably carried out at a temperature of about 40°.

The deposition of the copper in steps a) and c) can be carried out in electrochemical cells which are well known to those skilled in the art (ref 1). When the nontoxic electrolyte is used, various anodes can be employed. It is possible to use soluble or insoluble anodes or a combination of soluble and insoluble anodes.

As soluble anodes, preference is given to using anodes composed of a material selected from the group consisting of electrolytic copper, phosphorus-containing copper and copper alloys. As insoluble anodes, preference is given to using anodes composed of a material selected from the group consisting of platinated titanium, graphite, iridium-transition metal mixed oxide and special carbon material (“Diamond-Like Carbon” or DLC) or combinations of these anodes. Particular preference is given to using mixed oxide anodes composed of iridium-ruthenium mixed oxide, iridium-ruthenium-titanium mixed oxide or iridium-tantalum mixed oxide. Further insoluble anodes may be found in Cobley, A. J. et al. (The use of insoluble Anodes in Acid Sulphate Copper Electrodeposition Solutions, Trans IMF, 2001, 79 (3), pp. 113 and 114).

If insoluble anodes are employed, a particularly preferred embodiment of the process is obtained when the substrates to be provided with the copper layer, which represent the cathode, are separated from the insoluble anode by an ion-exchange membrane in such a way that a cathode space and an anode space are formed. In such a case, only the cathode space is filled with the nontoxic electrolyte. An aqueous solution containing only an electrolyte salt, e.g. potassium pyrophosphate, potassium carbonate, potassium hydroxide, potassium hydrogencarbonate or a mixture thereof, is preferably present in the anode space. As ion-exchange membrane, it is possible to use cationic or anionic exchange membranes. Preference is given to using membranes of Nafion having a thickness of from 50 to 200 μm.

The process of the invention and in particular the heat treatment between the two plating steps thus enables the carrier liquid of the electrolyte used to be removed to such an extent that it does not lead to blister formation or flaking during later heating of the parts. If, in contrast, the copper layer is applied to the zinc from a, for example, pyrophosphate-containing electrolyte without the heat treatment according to the invention of step b), the liquid which penetrates into the porous base material can no longer escape during later heating of the coated parts and, due to the vapour pressure produced, leads to blister formation or flaking in the coating. This was not to be expected from the prior art.

REFERENCES

-   -   (1) Praktische Galvanotechnik         -   Eugen G. Leuze Verlag, Saulgau, 6th edition, 2005.     -   (2) Technologie der Galvanotechnik         -   Gaida, Aβmann         -   Eugen G. Leuze Verlag, Saulgau, 2nd edition, 2008.     -   (3) Galvanotechnik         -   Nasser Kanani         -   Carl Hanser Verlag, Munich, 2nd edition, 2009.     -   (4) Kupferschichten         -   Nasser Kanani         -   Eugen G. Leuze Verlag, Saulgau, 1st edition, 2000.

EXAMPLE

The plating of zinc diecastings with copper is carried out using an electrolyte solution having the following composition:

-   300 g/l of potassium pyrophosphate -   30 g/l of copper pyrophosphate -   made up to 1 litre with water -   pH adjusted to 8 by means of methanesulphonic acid.

The zinc diecastings are plated in a barrel at 40° C. and a current density of 0.5 A/dm² for a time of 3 minutes. The parts are then rinsed, stored at 150° C. for a time of 30 minutes and, after cooling, plated in the same electrolyte bath for a further 2 hours.

On checking for blister formation at a temperature of 150° C. for 30 minutes in a drying oven, none of the parts displayed blisters in the coating. 

1. Process for the copper plating of zinc diecastings having a reduced tendency to blister formation, characterized by the steps, a) deposition of a first copper layer having a thickness of less than 1 μm from a pyrophosphate-containing copper electrolyte, b) rinsing and drying of the parts at elevated temperature and c) thickening of the first copper layer in a pyrophosphate-containing copper electrolyte to a thickness in the range from 10 to 20 μm.
 2. Process according to claim 1, characterized in that, in step a), the first copper layer is deposited to a thickness in the range from 0.1 to 0.5 μm.
 3. Process according to claim 1, characterized in that, in step b), the parts are stored at from 100 to 180° C. for a time of from 10 to 60 minutes. 